Conventionally, an internal combustion engine of a vehicle is equipped with a throttle valve for controlling intake air. In a conventional structure, the throttle valve is equipped to an intake passage through which the engine draws intake air into combustion chambers. In a conventional structure, the piston of the engine produces negative pressure when drawing intake air in an intake stroke, and the negative pressure is applied to a joint portion between the intake passage and the throttle valve.
In a turbocharged engine, a throttle valve may be located on the downstream side of a compressor of a turbocharger. In this configuration, the compressor produces a positive pressure to pressurize intake air and feeds the pressurized air through an intake passage into the throttle valve. Consequently, a joint portion between the intake passage and the throttle valve may be applied with the positive pressure. Consequently, leakage is concerned at the joint portion.